Detection of Permeability Variations by a Shallow Geothermal Technique

During a seven-month sampling period surface-water, ground-water, and shallow subsurface temperatures were measured at a well field in the Coastal Plain province in southeastern Pennsylvania in an attempt to delineate zones of relatively greater permeability in the underlying glacial outwash materials. Temperature measurements were made at monthly intervals using a tele-thermometer equipped with a thermistor. Subsurface temperatures were taken at a depth of six feet (183 cm) in a network of 66 access holes. In order to recognize perturbations in the subsurface thermal regime, theoretical conductive heat flow calculation: of the temperature variation expected to occur at a depth of six feet (183 cm) were compared to the observed temperature variation measured in the network of access holes. Isothermal maps, contouring equal temperatures at a specific time, and isallothermal maps, contouring equal temperature changes in a specific time interval, were constructed from the access hole data. These maps facilitated the identification of areas not conforming to the predicted value of annual subsurface temperature variation. Anomalous areas were found to represent regions where differing land-use practices or ground-water movement perturbed the normal thermal regime. Access hole temperatures measured in areas not covered by a dense vegetative cover or a tree canopy were found to have greater temperature variation than predicted. Areas where the temperature variation in the access holes was less than predicted were found to overlie known zones of relatively greater permeability in the underlying materials.